In a number of applications utilizing wideband amplifiers, it is often desirable to electronically control the gain of the amplifier, without significantly affecting any other performance parameter of the amplifier. This type of gain control, generally referred to as automatic gain control (AGC), is particularly useful in communication circuits, such as radio frequency (RF) and intermediate frequency (IF) amplifiers, to improve signal-handling capability and/or the dynamic range of the amplifier.
AGC is a well-known technique that automatically changes the gain of a circuit so that a desired output signal generated by the circuit remains essentially constant despite variations in input signal strength. In a simple AGC system, an example of which is shown in FIG. 1, the input signal is amplified by a variable gain amplifier (VGA) 102 whose gain is controlled by an external control signal, Vc. The output of the VGA is then fed to a detector 104 which senses one or more parameters of the output signal, such as amplitude, carrier frequency, index of modulation, etc. Any undesired component is filtered out and the remaining signal is compared (e.g., by comparison block 106) with a reference signal. The result of the comparison is used to generate the control signal Vc for adjusting the gain of the VGA so as to obtain the desired output signal.
In a wideband code division multiple access (WCDMA) application, an AGC loop is generally required to have a large dynamic range. For example, according to the third-generation wireless format (3G) specification, a receiver must be able to accommodate signal levels between −106.7 decibels referenced to a milliwatt (dBm) and −25 dBm. A step in power occurs, for example, when a WCDMA receiver makes measurements on the signal quality from neighboring base stations. One possible scenario is that a neighboring base station is transmitting at a different carrier frequency. The in-phase/quadrature (IQ) demodulator, sometimes referred to as a base-band demodulator, then changes carrier frequency for a short period of time which leads to an abrupt change in the received power. To minimize the measurement time, the AGC loop should compensate for the new received power level as fast as possible.
AGC bandwidth impacts the bit error rate (BER) performance of a system. If the AGC bandwidth is too low, then the system will not be able to sufficiently track power changes of the channel over which the system is communicating. If the AGC bandwidth is too high, then the system will exhibit an undesirable amount of gain noise. Generally, the desired AGC bandwidth of a communication system will depend upon, among other characteristics, the speed of motion of the handsets or other terminals used in the system. For instance, if a user is walking at a pedestrian speed of about 3 kilometers per hour (km/h), then the AGC bandwidth should be relatively narrow. Alternatively, if the user is driving in a car at a speed of about 120 km/h, then the AGC bandwidth should be relatively wide.
Unfortunately, in present WCDMA terminals the AGC bandwidth is typically set to be constant, perhaps based on an anticipated speed at which the WCDMA terminal is to be used. In this case, BER performance will degrade considerably when the terminal speed does not match some preset speed to which the fixed AGC bandwidth is meant to correspond.
Accordingly, there exists a need for AGC which does not suffer from one or more of the above-noted problems exhibited by conventional AGC methodologies.